Proximity detection devices are used in a wide variety of applications for determining the relative nearness or separation of an object or person relative to another object or person. An application of such devices that has recently acquired considerable public interest involves using such devices to allow a responsible individual, such as parent or guardian, to monitor the whereabouts of another person in the custody of the responsible individual. One example of a monitoring system that is intended to equip a responsible individual with the ability to monitor the whereabouts of another person, such as a child, is described in the U.S. Pat. No. 4,598,272, to Cox, the disclosure of which is incorporated herein.
In accordance with the system described in the Cox patent, proximity monitoring is performed by equipping each of a parent and a child with a respective radio transmitter--radio receiver pair. The radio transmitter carried by the child is operative to broadcast or radiate a high frequency RF electromagnetic wave to which the receiver in the unit carried by the parent is tuned. Should the level of the received RF signal monitored by the parent's radio receiver drop below a prescribed threshold, indicating that the child has moved to location beyond a prescribed distance from the parent, an alarm signal is generated by the parent's device. In response to the alarm signal, the parent may then active his or her own radio transmitter, which broadcasts a radio wave that has substantial signal strength, so that it will be detected by the child's unit. When the child's unit detects the RF signal transmitted by the parent's unit, it outputs an audible alarm signal that is loud enough to be heard by the parent, thereby enabling the parent to locate the child.
Now although the signal strength-monitoring--alarm--response radio wave communication scheme employed in the system described in the Cox patent facilitates alerting a parent/guardian of the separation of a child, patient, etc. beyond a prescribed distance, it has been found that the use of a high frequency radio wave as the signalling mechanism has a number of inherent problems that limit its performance, and thereby prevent its practical application to a wide variety of signalling environments.
More particularly, a fundamental shortcoming of a high frequency radio wave (e.g. one on the order of 300+ MHz) is the fact that radio waves are subject to multipath propagation, which can be especially severe in the interior of a building. Secondly, colinearity between the direction of polarization of the broadcast RF signal and the receiver antenna is required for ensuring optimum signal reception. A further problem is the effect of the dielectric distortion effect of the human body (a substantial salt water mass) on the signal, which can typically causing a fluctuation in signal amplitude on the order of 10-15 dB. Moreover, since the signal strength of a radiated electromagnetic wave is inversely proportional to the square of the distance from the emitter, the setting of a signal strength threshold to trigger an alarm yields very imprecise results, especially when considering the other effects described above.